The invention relates to a process of cleaning exhaust gases from combustion installations, in particular from diesel, injection and gas/diesel installations, large gas petrol engines, gas turbines and boiler installations which are fired by liquid, gas or solid fuels, through the introduction into the hot flow of exhaust gas of a dilute urea solution of differing concentrations prepared in a reagent tank and an at least single stage catalytic reaction of the toxic, gaseous exhaust gas constituents with a reduction stage or a reduction and an oxidation stage. The invention also relates to an exhaust gas cleaning installation with a reagent tank, a metering and feed device for the dilute urea solution and an electrical control and regulation unit to implement the process.
Catalysts in the exhaust gases of combustion installations are of increasing importance, and in many countries they constitute a necessary prerequisite in order to comply with legal standards.
Combustion installations with maximum levels of efficiency which simultaneously eliminate toxic gases by means of catalysts are the most environmentally friendly and energy-saving. The useful power (level of efficiency) extracted per kilogram of oil or per cubic meter of gas can be decisive in determining whether a system is sensible and environmentally friendly from an energy point of view. Maximum conversion rates of toxic chemicals can only be achieved by the use of catalyst technology.
Catalysts are divided into two main groups:
Three-way catalysts, which are used in operation without excess air, and are not of interest here. PA1 SCR (SCR=Selective Catalytic Reduction) catalysts, which are used in exhaust gas cleaning installations of combustion installations. High nitric oxide values at or below the limits required by the legislators can be achieved with SCR catalysts. For NOx reduction, a reagent is used in addition to a selective reduction catalyst. Ammonia is highly suitable as a reagent, although it is a problematic medium as regards transport, storage and handling. Urea is therefore also used as a substitute for ammonia. This is supplied as a white, dry granulate or as a ready-to-use solution, it is non-toxic, odour-free and poses no problems as regards storage and transport. PA1 the feed device is designed as a dual substance nozzle appliance and opens out into the pyrolysation channel, and comprises a reversing valve for the working and blowing out position, a casing tube for the compressed air arranged in the exhaust gas flow area, a urea conductor in the casing tube at a distance and a nozzle for fine spray of the dilute urea solution, PA1 at least two cross-blow mixers are fitted at a distance apart in the mixing channel, and PA1 at least one honeycomb reduction catalyst designed with lengthwise channels or at least one reduction catalyst and at least one oxidation catalyst are fitted in the reaction channel facing towards the exhaust gas flow. Special and further design forms of the exhaust gas cleaning installation are the subject of dependent patent claims. PA1 No waste products requiring disposal are formed, only non-toxic gases. PA1 Thanks to the dual substance nozzle appliance, the dilute urea solution may be injected in with maximum operating safety, in such a way that the urea decomposes completely. The compressed air acts simultaneously as a transport, cooling and blow-out medium. PA1 The catalysts are recyclable. They may be fitted and removed manually from the catalyst cases without lifting appliances. PA1 The installation may be ideally adapted to suit the spatial conditions, since it is designed extended or compact, horizontal, vertical or diagonal. PA1 The exhaust gases of all combustion processes in engines, boilers and turbines which work with excess oxygen may be cleaned. PA1 The injection appliance operates in a low to medium pressure range, the dilute urea solution is supplied with low conveyance pressure, the compressed air for cooling and spraying is supplied with an excess pressure of only 0.2 to 8 bars. PA1 The injection appliance automatically switches over to self-cleaning with compressed air through the operation of a three-way valve, in the event of a blockage or when work is complete.
In DE,A1 3830045, a process is proposed for the reduction of nitric oxides contained in exhaust gases, in particular those of diesel machines, under oxidation conditions by means of a catalyst containing zeolite, whereby a substance containing urea is added to the exhaust gas as a reduction agent. Metering of the reduction agent may be stoichiometric, under or over-stoichiometric. DE,A1 3830045 aims to create a way of reducing nitric oxides in exhaust gases, avoiding problematic reduction agents, using a catalyst which contains zeolite even where the exhaust gas has a low hydrocarbon content. The catalyst which contains zeolite is therefore of fundamental significance for DE,A1 3830045 corresponding to U.S. Pat. No. 4,946,659.